1. Field of the Invention
This invention relates to ultrahigh carbon steels and methods of thermomechanical processing ultrahigh carbon (UHC) steels such that the steels are fine grained and spheroidized.
2. Description of the Prior Art
A class of steels known as ultrahigh carbon (UHC) steels has been developed by Sherby et al. and described in U.S. Pat. No. 3,951,697 issued Apr. 20, 1976. These UHC steels are typically plain carbon steels containing between 1.0-2.1% carbon by weight, although they can contain small alloying additions (&lt;2%) of elements such as Cr, Si, V, etc. Conventional steels contain between 0.1 and 0.8% C and cast irons contain over 2.1% C. Thus, UHC steels are intermediate in carbon content between the two groups of iron-based materials.
These UHC steels have been processed to have a fine-grained microstructure containing spheroidized cementite particles. In this condition the steels are superplastic in the temperature range 600.degree.-850.degree. C. Superplasticity is the ability of certain materials to undergo extensive neck-free elongations. This property can be utilized in the manufacture of complex shapes. In addition, because of their fine structure, UHC steels can be bonded readily to themselves or other ferrous alloys in the solid-state at temperatures that are much lower than those used commercially for bonding of steels. As is generally found with other fine structures, at room temperature, UHC steels have good strength, ductility and toughness. Finally, the UHC steels can be heat treated by quenching, and, because of the high carbon content, extremely-high hardnesses can be developed.
The Sherby patent teaches the desirability of having the cementite in the ferrite-cementite region in spheroidized form rather than in lamellar form. In that patent, a number of thermomechanical processing techniques are described to accomplish the formation of spheroidized cementite particles in fine-grained ferrite. These techniques involve a homogenization step in which, by heating the steel into the single-phase austenite region, austenite having a uniform carbon content is created. Following the homogenization, a number of techniques are described to refine the iron grain and obtain cementite in spheroidized form. These techniques all require large strains during warm working in the ferrite-cementite region to develop the desired structure. In addition to the requirement of large strains, all of the techniques described in the patent also require isothermal deformation at some point in the processing. These requirements are undesirable because of the uneconomical aspect of such practices. No consideration is given to the utilization of the divorced eutectoid transformation in the processing techniques described in the Sherby patent.
The divorced eutectoid transformation, in which the eutectoid transformation proceeds by the formation of spheroidized carbides and ferrite instead of lamellar pearlite, was observed by Honda and Saito in 1920 in an academic investigation. In another more recent study, Nakano et al. have described the "Effects of Chromium, Molybdenum and Vanadium on Spheroidization of Carbides on 0.8% Carbon Steel" (Transaction ISIJ, Vol. 17, 1977). They showed that the divorced eutectoid transformation could occur in their steels upon slow cooling (20.degree. C./hour) from above the A.sub.1 transformation temperature. They emphasized that the degree of spheroidization was a function of austenitizing temperature and the authors were primarily only interested in the influence of Cr, Mo and V on the degree of spheroidization. No mechanical working was carried out concurrently with the DET. In addition, Nakano et al. worked only with steels having carbon contents significantly less than UHC steels. The major difference between the UHC steels and 0.8% C steels is the large amount of pro-eutectoid carbide present in UHC steels. This pro-eutectoid carbide has to be processed into spheroidized form in order to obtain the desired final microstructure claimed in this application.